5:15 PM - 7:15 PM
[SGD02-P06] Spatial distribution of ellipsoidal height of mean sea level along the coast of Japan toward the introduction of ellipsoidally referenced survey
Keywords:Ellipsoidally referenced survey (ERS), mean sea level, Hydrographic survey, satellite positioning, geoid, sea surface height
Hydrographic and Oceanographic Department, together with Port and Harbor Bureau, is working to introduce ellipsoidally referenced survey (ERS), a new method of measuring water depth. To introduce ERS, chart datum based on the ellipsoid that represents the actual sea level height is required. In this study, the ellipsoidal height of the mean sea level at tidal stations around coast of Japan was calculated, which can be the primary data for chart datum on the ellipsoid.
The mean sea level obtained from long-term observations does not basically coincide with Tokyo Peil (T.P.) due to the influence of ocean currents and long-term weather. Yoritaka and Hanawa (2020) reported the elevation (height referenced to T.P.) of the mean sea level along the Japanese coast using the results of leveling surveys, Japanese Geodetic Datum 2000. According to this report, the elevations (T.P.) of mean sea level on the northwest coast of Honshu were about 20 cm higher than those on the east coast of Honshu. The elevation (T.P.) can be considered as the difference between the ellipsoidal height and the geoid. The difference of about 20 cm is too large to be ignored in creating chart datum for hydrographic survey. Therefore, in calculating the distribution of the ellipsoidal height of the mean sea level, the difference from the geoid should be taken into account.
However, leveling surveys from the Japanese datum of leveling adopted by Yoritaka and Hanawa (2020) and Isoda and Yamaoka (1991) can have cumulative errors derived from the long distances, and also need complex correction for ground movement.
In this study, the ellipsoidal height of the mean sea level along the coast of Japan was calculated by using satellite positioning, and by applying simple correction for ground movement. First, mean sea levels observed at tidal stations were converted to an ellipsoidal reference by GNSS static observations conducted for more than 6 hours or GNSS continuous observation points installed at tidal stations. Next, in the case of GNSS static observations, the ground movement was corrected using the amount of vertical change of the F5 solution of nearby GPS reference station.
The resulting distribution of mean sea level, which was converted to the height from the geoid model of GSIGEO2011(Ver2.2), showed about 25 cm along the coast of Japan Sea and about 0 cm along the east coast of Honshu. The results were consistent with those reported by Yoritaka and Hanawa (2020). On the other hand, from the south coast of Honshu to the east coast of Kyushu, mean sea levels above the geoid were a few centimeters to 10 cm larger than those reported by Yoritaka and Hanawa (2020). The difference was considered to be caused by the meandering path of the Kuroshio Current.
It was found that the simple correction for ground movement using the F5 solution was effective in calculating the ellipsoidal height of the mean sea level. However, In the GNSS static observation, errors of a few centimeters due to multipath were observed.
To improve the accuracy, extending the GNSS observation time and refining the correction for ground movement will be considered. In addition, since the height of the mean sea level varies spatially, the horizontal gradient should be taken into account in the open ocean and inner bay to create horizontally seamless chart datum. In the open ocean, in addition to observations by Autonomous Ocean Vehicle (AOV) and GPS buoys, we are planning to use a hydrodynamic model that assimilates satellite altimeters, which is also effective in inner bays.
The Geospatial Information Authority of Japan has announced that the national geodetic datum will be replaced by Japanese Geodetic Datum 2024 (JGD2024). JGD2024, based on satellite positioning, will enable us to use elevation of benchmarks with small errors derived from leveling survey and ground movement. It is expected that the accurate elevation and ellipsoidal height of mean sea level will be obtained by using JGD2024.
The mean sea level obtained from long-term observations does not basically coincide with Tokyo Peil (T.P.) due to the influence of ocean currents and long-term weather. Yoritaka and Hanawa (2020) reported the elevation (height referenced to T.P.) of the mean sea level along the Japanese coast using the results of leveling surveys, Japanese Geodetic Datum 2000. According to this report, the elevations (T.P.) of mean sea level on the northwest coast of Honshu were about 20 cm higher than those on the east coast of Honshu. The elevation (T.P.) can be considered as the difference between the ellipsoidal height and the geoid. The difference of about 20 cm is too large to be ignored in creating chart datum for hydrographic survey. Therefore, in calculating the distribution of the ellipsoidal height of the mean sea level, the difference from the geoid should be taken into account.
However, leveling surveys from the Japanese datum of leveling adopted by Yoritaka and Hanawa (2020) and Isoda and Yamaoka (1991) can have cumulative errors derived from the long distances, and also need complex correction for ground movement.
In this study, the ellipsoidal height of the mean sea level along the coast of Japan was calculated by using satellite positioning, and by applying simple correction for ground movement. First, mean sea levels observed at tidal stations were converted to an ellipsoidal reference by GNSS static observations conducted for more than 6 hours or GNSS continuous observation points installed at tidal stations. Next, in the case of GNSS static observations, the ground movement was corrected using the amount of vertical change of the F5 solution of nearby GPS reference station.
The resulting distribution of mean sea level, which was converted to the height from the geoid model of GSIGEO2011(Ver2.2), showed about 25 cm along the coast of Japan Sea and about 0 cm along the east coast of Honshu. The results were consistent with those reported by Yoritaka and Hanawa (2020). On the other hand, from the south coast of Honshu to the east coast of Kyushu, mean sea levels above the geoid were a few centimeters to 10 cm larger than those reported by Yoritaka and Hanawa (2020). The difference was considered to be caused by the meandering path of the Kuroshio Current.
It was found that the simple correction for ground movement using the F5 solution was effective in calculating the ellipsoidal height of the mean sea level. However, In the GNSS static observation, errors of a few centimeters due to multipath were observed.
To improve the accuracy, extending the GNSS observation time and refining the correction for ground movement will be considered. In addition, since the height of the mean sea level varies spatially, the horizontal gradient should be taken into account in the open ocean and inner bay to create horizontally seamless chart datum. In the open ocean, in addition to observations by Autonomous Ocean Vehicle (AOV) and GPS buoys, we are planning to use a hydrodynamic model that assimilates satellite altimeters, which is also effective in inner bays.
The Geospatial Information Authority of Japan has announced that the national geodetic datum will be replaced by Japanese Geodetic Datum 2024 (JGD2024). JGD2024, based on satellite positioning, will enable us to use elevation of benchmarks with small errors derived from leveling survey and ground movement. It is expected that the accurate elevation and ellipsoidal height of mean sea level will be obtained by using JGD2024.